The goals of the propposed research are to determine the effect of age on the syngeneic mixed lymphocyte reaction (MLR) and to investigate the function of the syngeneic MLR in vivo. Specifically, the capacity of lymphocytes from mice of different ages to respond and to stimulate in the syngeneic MLR will be compared. The cellular basis of the impaired syngeneic MLR observed in very young or old mice will be determined. Finally, the role of the syngeneic MLR in cell-mediated immunity will be determined. Mice will be treated with a monoclonal antiserum specific for the subset of T cells which respond in the syngeneic MLR in order to deplete these animals of this subpopulation of T cells. The capacity of these animals to generate cytotoxic T cells, suppressor activity or delayed hypersensitivity will be determined.